/* 

反转一个单链表。

示例:

输入: 1->2->3->4->5->NULL
输出: 5->4->3->2->1->NULL
进阶:
你可以迭代或递归地反转链表。你能否用两种方法解决这道题？

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/reverse-linked-list
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

*/
/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */

// 迭代
var reverseList = function (head) {
    if (head === null || head.next === null) return head;
    let pre = null;
    let cur = head;
    while (cur) {
        const next = cur.next;
        cur.next = pre;
        pre = cur;
        cur = next;
    }

    return pre;
};

// 递归1
var reverseList = function (head) {
    const reverse = function (pre, cur) {
        if (!cur) return pre;
        const next = cur.next;
        cur.next = pre;
        return reverse(cur, next);
    }

    return reverse(null, head);
};

// 递归2
var reverseList = function (head) {
    if (head === null || head.next === null) return head;
    let newHead = reverseList(head.next);
    head.next.next = head;
    head.next = null;

    return newHead;
};